His research has shown that this protective layer is weakened in people with a rare genetic skin disease who develop eczema or other allergies.

Penetrating the skin

The UCL team has also shown that if the outer protective layer of skin is stripped away - using something as simple as sellotape - allergy-causing particles are able to penetrate the skin.

They are then taken up by specialised cells called Langerhans cells, which are found in a layer of the skin called the epidermis.

The Langerhans cells then move from the skin to the local lymph nodes of the immune system, and induce an allergic response.

Professor Callard said: "Despite its popularity over the past 20 years there is very little supporting evidence for the "hygiene hypothesis".

"In contrast, there is mounting evidence from both studies of rare genetic conditions and our lab work to support an important role for abnormalities in the outer protective layer of the skin in allowing allergic sensitization."

Professor John Harper, who also worked on the study, said people had nothing to fear from normal standards of hygiene - the potential problem was over-zealous use of harsh cleaning products.

He said: "Good standards of hygiene are clearly important to prevent spreading of unpleasant diseases.

"But if this proposed mechanism does turn out to be true, we may be able to reduce the incidence of these diseases by developing new treatments which specifically target the outer protective layer of the skin."

Margaret Cox, of the National Eczema Society, said: "Patients with atopic eczema have especially sensitive skin.

"Because soap and biological detergents de-grease the skin, if you are genetically predisposed to eczema you would be well advised to avoid using such products and switch to less abrasive and more nourishing emollients."

Over the last 30 years the prevalence of eczema in the UK has increased three fold.